This invention relates to electric rotating machines and more particularly to electric motors of the type in which the rotor of the motor turns in discrete increments or steps.
There has been developed a class of alternating current synchronous and stepper motors which exhibit extremely rapid starting characteristics and are highly efficient. Included in this class are motors utilizing a permanent magnet rotor having a plurality of alternating poles and annular stators mounted side by side in tandem with a plurality or salient stator poles on their inner peripheries. The windings of each of the stators are independently energizable out of phase with one another so that the rotational direction of the motor can be controlled.
While motors of this class are satisfactory for many purposes, such motors have exhibited certain disadvantages. For example, because of the tandem arrangement of the energizing windings, such motors are not readily adaptable for applications which require a motor having a very low profile, or a pancake-type construction, such as may be required of motors for computer disk drives or automobile electric windows. In addition, because there are structural limitations as to how thin the magnetic rotor can be, the thickness, and hence the weight, inertia and cost, of the permanent magnet rotor of prior art tandem stator motors may be as much as twice as great as magnetically required to develop full torque. Furthermore, such motors have a considerable amount of wasted space, making such motors undesirable where compact construction is needed.
It is therefore an object of the present invention to provide a motor having a low profile, pancake-type construction wherein the magnetic rotor is efficiently utilized magnetically to provide increased torque with no increased weight of the rotor and which is very compact in construction with a minimum of unused space.